The Applicants disclosed such an apparatus in French Patent No. PV 144 111 of Mar. 15, 1968 entitled: "Apparatus for measuring continuous high voltage current with high dynamics".
That apparatus mainly comprises an adapter of symmetrical input and very high impedance, an amplification chain coupled to the output of the adapter and associated with a linear feedback chain and a logarithmic feedback chain comprising at least one first logarithmic element; a second temperature compensating logarithmic element identical with the first one and associated with the amplification chain, a reading device, at the output of the circuits of the amplification chain, at least one threshold circuit coupled to the output of the amplification chain and the input of a range switching members control circuit via a lockout gate whose input is coupled to a delay circuit, and calibrating and shifting elements of the reading device and associated with the amplification chain, the switching members causing at least the switching of the linear feedback chain at the level of the threshold circuit threshold.
It is known to use as the adapter with symmetrical input and high impedance, for measuring apparatuses of the kind specified, a symmetrical assembly of two transistors with insulated control electrodes, associated with two ordinary transistors. Such an assembly is disclosed in Certificate of Addition No. PV 135 544 filed by the Applicants on Jan. 10, 1968 and entitled: "Apparatus for the logarithmic measurement of quantities of charges delivered by an ionization chamber".
FIG. 1 is a diagram illustrating an embodiment of a low current preamplifier according to the teaching of the two Patents cited.
The preamplifier mainly comprises an impedance adapter 2, an amplifier 4 and a symmetrical stage of two transistors 6, to ensure an adequate output current. The preamplifier receives at the input the low current to be measured coming, for example, from an ionization chamber. The current is, for example, between 10.sup.-14 and 10.sup.-11 A. The output current delivered by the amplifier is of the order of 5 mA.
The impedance adapter 2 is protected at the input by an assembly 10 of diodes and a network RC comprising a high value resistor 12 and a capacitor 14. Because of its symmetrical structure, the impedance adapter 2 is temperature-stable.
It is difficult to amplify a very low input current. This requires an impedance adapter 2 having the lowest possible leakage current (low current to be measured), and a feedback chain 8 of high precision and high stability. The non-inverting input of the amplifier has a means 18 for zero point adjustment of the amplifier 4 in the absence of an input signal ("offset" control).
The feedback chain 8 of the amplifier comprises two paths in parallel of different sensitivity. The value of the resistance of each path determines the gain of the preamplifier. The path of higher sensitivity comprises a very high value resistor 20. The path of lower sensitivity comprises a resistor 22 of lower value, for example, at least 100 times lower. Capacitors 24 and 26 are disposed in parallel with the resistors. The lowest sensitivity is obtained by connecting the two paths in parallel. This is done by a switch 28 controlled by a high insulation relay 30.
It is an object of the invention to improve the measuring apparatus disclosed in the French Patent cited hereinbefore. This improvement relates mainly to the preamplifier of the measuring apparatus.